[Yago or E-a'-go]. Othello's ensign or ancient. He hated the Moor both because Cassio, a Florentine, was preferred to the lieutenancy instead of himself, and also from a suspicion that the Moor had tampered with his wife; but he concealed his hatred so well that Othello wholly trusted him. Iago persuaded Othello that Desdemona intrigued with Cassio, and urged him on till he murdered his bride. His chief argument was that Desdemona had given Cassio a pocket-handkerchief, the fact being that Iago had set on his wife to purloin it. After the death of Desdemona, Emilia (Iago's wife) revealed the fact, and Iago was arrested.
Shakespeare generally makes three syllables of the name, as -
Let it not gall your patience, good I-a-go. Left in the conduct of the bold I-a-go. ii.2. 'Tis one I-a-go, ancient to the general.
Iambic Father of Iambic verse. Archilochos of Paros (B.C. 714-676).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894